THIS INVENTION relates to the production of hydrocarbon products. It relates in particular to an integrated process for producing hydrocarbon products and energy.
According to a first aspect of the invention, there is provided an integrated process for producing hydrocarbon products and energy, which process includes
reforming a hydrocarbonaceous gaseous feedstock to synthesis gas;
exothermally reacting the synthesis gas at elevated temperature and pressure, and in the presence of a Fischer-Tropsch catalyst, to produce a range of hydrocarbon products of differing carbon chain lengths:
controlling the reaction temperature by indirect heat exchange of a reaction medium, comprising synthesis gas and hydrocarbon products, with water, with the water being converted to steam (xe2x80x98FT steamxe2x80x99);
burning a combustible gas in a combustion chamber of a gas turbine generator, to form combusted gas, and expanding the combusted gas through an expansion chamber of the gas turbine generator to form hot flue gas, while generating electrical energy by means of the gas turbine generator; and
superheating at least some of the FT steam by means of at least some of the hot flue gas, thereby producing superheated FT steam.
The FT steam may be at a medium pressure between about 800 kPa(a) and about 3000 kPa(a)
The reforming of the hydrocarbonaceous gaseous feedstock to synthesis gas may be effected in a synthesis gas production stage. The synthesis gas comprises at least CO, H2 and CO2, and is at an elevated temperature.
The process may then include, prior to reacting the synthesis gas, cooling the synthesis gas by indirect heat exchange with water, with the water being converted to steam (xe2x80x98Syngas steamxe2x80x99).
The process may also include feeding the cooled synthesis gas, as a feedstock, to a hydrocarbon synthesis stage in which the exothermal Fischer-Tropsch reaction of the synthesis gas is effected. A vapour phase comprising light hydrocarbon products and unreacted synthesis gas, a liquid phase comprising heavier liquid hydrocarbon product, and an aqueous phase comprising water and any soluble organic compounds formed during the reaction of the synthesis gas, may be produced in the hydrocarbon synthesis stage. The vapour phase, the liquid phase and the aqueous phase may then be withdrawn from the hydrocarbon synthesis stage.
The gas turbine generator constitutes, or forms part of, an electricity generation stage. The hot flue gas is thus withdrawn from the electricity generation stage.
The superheating of the FT steam may thus be effected in a heat exchange stage. If desired, high pressure steam (xe2x80x98HP steamxe2x80x99) having a pressure between 3000 kPa(a) and 12000 kPa(a) may also be generated in the heat exchange stage by means of hot flue gas. The HP steam may, if desired, be superheated. When HP steam is generated, a portion thereof may, if desired, be used a process steam in the hydrocarbon synthesis stage.
The process may include feeding at least some of the superheated FT steam into an energy generation stage, which may comprise a steam turbine. The superheated FT steam is then directed into the steam turbine which thereby generates electrical and/or mechanical energy.
Thus, according to a second aspect of the invention, there is provided an integrated process for producing hydrocarbon products and energy, which process Includes
in a synthesis gas production stage, reforming a hydrocarbonaceous gaseous feedstock to synthesis gas comprising at least CO, H2 and CO2, with the synthesis gas being at elevated temperature, and cooling the synthesis gas by indirect heat exchange with water, with the water being converted to steam (xe2x80x98Syngas steamxe2x80x99):
feeding tho cooled synthesis gas, as a feedstock, to a hydrocarbon synthesis stage;
in the hydrocarbon synthesis stage, exothermally reacting the synthesis gas at elevated temperature and pressure, and in the presence of a Fischer-Tropsch catalyst, to produce a range of hydrocarbon products of differing carbon chain length; controlling the reaction temperature by indirect heat exchange of a reaction medium comprising the synthesis gas feedstock and the hydrocarbon products with waters with the water being converted to steam (xe2x80x98FT steamxe2x80x99); and producing a vapour phase comprising light hydrocarbon products and unreacted synthesis gas, a liquid phase comprising heavier liquid hydrocarbon products, and an aqueous phase comprising water and any soluble organic compounds formed during the reaction of the synthesis gas;
withdrawing the vapour phase, the liquid phase and the aqueous phase from the hydrocarbon synthesis stage;
in an electricity generation stage comprising a gas turbine generator, burning a combustible gas in a combustion zone or chamber of the gas turbine generator, to form combusted gas, and expanding the combusted gas through an expansion chamber of the gas turbine generator to obtain hot flue gas, with electrical energy being generated by the gas turbine generator;
withdrawing the hot flue gas from the electricity generation stage;
in a heat exchange stage, using the hot flue gas to superheat at least some of the FT steam and/or to generate high pressure steam (xe2x80x98HP steamxe2x80x99) having a pressure between 3000 kPa(a) and 12000 kPa(a) and, optionally, superheating the HP stream;
feeding at least some of the superheated steam into an energy generation stage comprising, for example, a steam turbine;
when the HP steam is generated, optionally using a portion thereof as process steam in the hydrocarbon synthesis stage;
in the energy generation stage, generating electrical and/or mechanical energy by meant of the steam turbine into which the superheated steam is directed.
The reforming of the hydrocarbonaceous gas, ie of the hydrocarbonaceous gaseous feedstock, to synthesis gas is thus effected by reacting the hydrocarbonaceous gas with steam and/or oxygen at high temperature, ie high temperature reforming is employed. Typically, the conversion may be effected by means of steam reforming, which does not require the use of oxygen, autothermal reforming, in which the hydrocarbonaceous material reacts with oxygen in a first reaction section, whereafter an endothermic steam reforming reaction takes place adiabatically in a second reaction section; ceramic oxygen transfer membrane reforming, in which oxygen required for the reforming reaction is transported through an oxygen permeable membrane into a reaction zone; plasma reforming in which the reforming reaction is driven by an electrically generated plasma; non-catalytic partial oxidation; or catalytic partial oxidation. If desired, two or more of these conversion mechanisms or technologies may be combined, eg to optimize thermal efficiency, or to obtain an optimized or beneficial synthesis gas composition. A lower temperature prereforming step may be employed before the high temperature reforming takes place, and is particularly useful for preventing carbon formation by thermal decomposition when higher carbon number hydrocarbons are present in the feedstock.
The hydrocarbonaceous gaseous feedstock may, in particular, be natural gas, or a gas found in association with crude oil, and which comprises mainly CH4 and other hydrocarbons. An initial cooling step may be used to knock out condensable hydrocarbons prior to the gas being subjected to the reforming. The synthesis gas will then contain, in addition to CO, H2 and CO2, also some unreacted CH4 and inert gases.
The oxygen may be obtained from a cryogenic air separation plant in which air is compressed and separated cryogenically into oxygen and is nitrogen. At least a portion of the electrical energy and/or the mechanical energy produced in the electricity generation stage and/or in the energy generation stage, may be used as a power source for said cryogenic air separation plant.
The process may include preheating the hydrocarbonaceous gaseous feedstock prior to feeding it into the reformer. Typically, it may be preheated then in excess of 400xc2x0 C. The preheating may be affected in a gas fired furnace, which may be fired using a portion of the hydrocarbonaceous gas and/or a portion of the vapour phase produced in the hydrocarbon synthesis stage.
The synthesis gas produced in the synthesis gas production stage is typically at an elevated temperature in excess of 800xc2x0 C., and the generation of the Syngas steam using the hot synthesis gas may be effected in a heat recovery unit. The Syngas steam may also be at a high pressure between about 3000 kPa(a) and about 12000 kPa(a).
The Syngas steam may be used in the process, eg where applicable, as a reactant in the reforming reaction; for heating and/or in a steam turbine to produce electrical and/or mechanical energy for driving compressors such as an air compressor in the cryogenic air separation plant and for driving pumps and other equipment. However, in one embodiment of the invention, at least a portion of the Syngas steam may be converted to superheated steam by heat exchange with hot flue gas from the electricity generation stage, as described in more detail hereinafter.
As indicated hereinbefore, at least a portion of the Syngas steam may be used in a steam turbine. It is well known from engineering practice that higher efficiencies can be obtained in steam turbines if the steam employed therein has a certain degree of superheat. Superheated high pressure steam may be generated by heat exchange of the Syngas steam with the hot synthesis gas but this results in less steam in total being produced from the anthalpy in the synthesis gas. Such process superheaters are also expensive due to the non-standard materials of construction required to avoid metal dusting. Thus, the superheating may be done externally using a gas fired furnace, such as the furnace used for preheating the gas feedstock to the synthesis gas production stage.
In the hydrocarbon synthesis stage, the hydrocarbon products produced have chain lengths varying from 1 carbon atom to over 100 carbon atoms. The hydrocarbon synthesis stage may include a suitable reactor such as a tubular fixed bed reactor, a fluidised bed reactor, a slurry bed reactor or an emulating bed reactor, in which the hydrocarbon products are produced. The pressure in the reactor may be between 1000 and 10000 kPa. The reactor will thus contain the Fischer-Tropsch catalyst, which will be in particulate form. The catalyst may contain, as its active catalyst component, one or more of Fe, Co, Ni, Ru, Re and/or Rh.
As indicated hereinbefore, the reaction, ie the Fischer-Tropsch reaction, is exothermic, and occurs at a temperature between 200xc2x0 C. and 380xc2x0 C. The reactor temperature is controlled, eg the reactor may be maintained at near-isothermal condition, by passing water as a cooling medium through the reactor, with the water thus being converted into the FT steam and thereby removing the heat of reaction. For a tubular fixed bed reactor, the water usually passes on the shell side of the reactor, while the hydrocarbon synthesis occurs inside the tubes. For the other types of reactors, coils are normally located inside the reactor, with the water passing through the coils. Although the pressure of the FT steam that is generated can vary depending on the desired temperature of the hydrocarbon synthesis reaction, the FT steam so produced is typically at a lower pressure than the Syngas steam. The FT steam is thus typically at medium pressure, is at a pressure between about 800 kPa(a) and about 3000 kPa(a), as hereinbefore described.
In accordance with the Fischer-Tropsch reaction, CO and H2 are converted into hydrocarbon products according to the following generalised formula 1);
nCO+(2n+1)H2xe2x86x92CnH2n+2+nH2Oxe2x80x83xe2x80x83(1)
As indicated hereinbefore, the Fischer-Tropsch reaction takes place at a temperature between 200xc2x0 C. and 380xc2x0 C., typically between 200xc2x0 C. and 350xc2x0 C. Lower temperature operation (200xc2x0 C. to 300xc2x0 C.) results in longer chain hydrocarbon formation containing varying quantities of olefins, alcohols and paraffinic compounds, and typically is effected in a fixed bed or bubble column/slurry bed reactor. Higher temperature operation (300xc2x0 C. to 350xc2x0 C.) produces a lighter product spectrum and more typically is effected in a fluidized bed reactor.
In all cases, the reaction is highly exothermic, with an approximate heat of reaction of 165 kJ/kmol of CO converted. In order to keep conditions in the reactor close to isothermal, heat must be removed from the reactor by heat exchange with the water and the generation of FT steam as hereinbefore described.
The process may include withdrawing the liquid phase and an overheads vapour phase separately from the reactor, feeding the overheads vapour phase into a product condensation unit, and withdrawing the vapour phase, the aqueous phase, and a condensed product phase, from the product condensation unit.
In the electricity generation stage, the combustible gas that is burned in the combustion chamber of the gas turbine generator may comprise a hydrocarbon gas component admixed with an oxygen containing gas. The hydrocarbon gas component may comprise the same hydrocarbonaceous gas as Is used as feedstock to the synthesis gas production stage, at least a portion of the vapour phase produced in the hydrocarbon synthesis stage, or mixtures thereof. The oxygen containing gas may be air, which may be compressed before it is admixed with the hydrocarbon gas component.
At least a portion of the vapour phase may thus be routed to the gas turbine generator as the hydrocarbon gas component, or as part of the hydrocarbon gas component. However, the process may instead, or additionally, include recycling at least a portion of the vapour phase to the synthesis gas production stage, so that it forms part of the hydrocarbonaceous gaseous feedstock to the synthesis gas production stage. Instead, or additionally, at least a portion of the vapour phase may be recycled to the hydrocarbon synthesis stage, so that it forms, together with the synthesis gas, the feedstock to the hydrocarbon synthesis stage. If desired, at least part of the vapour phase, for example any residual part thereof not required in the gas turbine generator or in the synthesis gas production stage or in the hydrocarbon synthesis stage, may be used as fuel gas in a fuel gas fired furnace, or used as a fuel gas in a power generation stage.
If desired, the aqueous phase may be treated further, to recover any organic components present therein.
The process may include feeding the liquid phase to a product work-up stage. In the product work-up stage, liquid hydrocarbon products may be upgraded by reaction with hydrogen at elevated temperature and pressure, to produce primarily diesel and naphtha hydrocarbon species. More specifically, in the product work-up stage, unsaturated hydrocarbons and oxygen components may be hydrogenated, in a single reactor or in a series of reactors, by reaction with hydrogen, while heavier hydrocarbon fractions may be cracked and isomerised. The product of the hydrotreatment reactor(s) may be sent to a series of distillation columns for separation into various fractions, such as naphtha, diesel and lubricating oil fractions.
Instead of subjecting the liquid phase to hydroprocessing, paraffins, olefin and/or alcohols may be extracted therefrom and subjected to further processing to produce a variety of chemical products therefrom. Instead, or additionally, the liquid phase may be subjected to catalytic reforming/platforming or fluidised catalytic cracking to convert the hydrocarbon products into aromatics and gasoline components.
In yet another version of the invention, the liquid fraction need not be subjected to further work-up, eg when it is in the form of a synthetic crude fuel. Such a crude fuel can then be mixed with crude oil, and subjected to processing as bulk crude oil in an oil refinery.
In the heat exchange stage, both HP steam, and medium pressure steam, ie steam at a pressure between 800 kPa(a) and 3000 kPa(a) (hereinafter also referred to as xe2x80x98MP steamxe2x80x99), may be produced. Either the HP steam or the MP steam, or both, may be superheated.
The steam turbine of the energy generation stage may be a two (or more) stage steam turbine generator. Superheated HP steam may be directed into a first stage of the generator, where it is expanded to a lower pressure, with this steam then being directed, together with superheated MP steam, into a second stage of the generator. In both stages of the steam generator, electrical and/or mechanical energy is thus generated. The superheated HP steam is thus, in the first stage expanded to about the same pressure as the superheated MP steam. The combined steam stream is, in the second stage, expanded to a suitable pressure, and may then be condensed or directed to a third lower pressure stage.
The process may, in one embodiment of the invention, include superheating the FT steam in the heat exchange stage, ie a by indirect heat exchange with hot flue gas. Thus, in this embodiment of the invention, none of the Syngas steam is superheated in the heat exchange stage In one version, only the FT steam may then be converted into the superheated MP steam used in the second stage of the steam turbine generator, with no additional MP steam being produced from boiler feed water (hereinafter referred to as xe2x80x98BFWxe2x80x99), ie none of the BFW is converted into superheated MP steam, with only HP steam being generated from additional BFW in the power generation flue gas heat recovery unit; however, in another version, some of the BFW may also be converted to superheated MP steam. The MP steam generated from the BFW by heat exchange with flue gas and the FT steam may then be mixed together prior to being superheated; however, instead, they may be superheated separately and then combined, before being directed into the second stage of the steam turbine generator.
However, in another embodiment of the invention, the process may include superheating a portion of the Syngas steam in the heat exchange stage, ie by heat exchange with hot flue gas, in addition to, or instead of, superheating the FT steam. Typically, all of the Syngas steam can then be superheated in the heat exchange stage. This avoids the necessity of having to superheat the Syngas steam in a separate fuel gas fired furnace. The fuel gas can instead, if desired, be used as a portion of the hydrocarbon gas component in the gas turbine generator. Instead, the fuel gas can be used for supplemental firing in the heat exchange stage to generate the superheated Syngas steam. For example, this supplemental firing may be effected in a convective section of an exhaust of the gas turbine, which thus constitutes a portion of the heat exchange stage.
The Syngas steam may be mixed with HP steam produced from BFW in the heat exchange stage, and the resultant combined HP steam stream then superheated in the heat exchange stage.
The process may include using a portion of the superheated HP steam to drive the air compressor of the gas turbine generator, while the remainder of the superheated HP steam if directed to the steam turbine generator. Instead, however, all of the superheated HP steam may be directed into the steam turbine generator.
If desired, preheating of the feedstock to the synthesis gas production stage can be effected in the convective section of the gas turbine generator exhaust, using hot flue gas, instead of in a separate gas fired furnace.
In yet another embodiment of the invention, the HP steam may be split, with a first portion thereof being directed, after superheating thereof, to the first stage of the steam turbine generator, as hereinbefore described, while a second portion thereof is routed to the synthesis gas production stage, where it is mixed with the Syngas steam. The mixed high pressure steam stream may then be used as process steam so that it is used in the reforming reaction. This is an efficacious approach in the event that insufficient high pressure steam is generated solely in the waste heat recovery section of the reformer for all process requirements. Such a case may exist for instance when steam reforming alone or a combination of reforming techniques is employed. For example, in a first step, endothermic steam reforming may take place, while in a second step oxygen burning autothermal reforming may occur. Steam reforming typically employs a steam/reformable carbon ratio of greater than 1.5, whereas autothermal reforming typically operates with this ratio below 1.5. A favoured arrangement for this combination of reformers is that the heat available in the autothermal reformer""s exit stream is used to supply the necessary energy to drive the endothermic steam reforming reaction that occurs in the steam reforming. This arrangement results in less energy being available in the waste heat recovery section to generate steam and also results in a higher total demand for process steam. Consequently the synthesis gas production stage can become a net consumer of steam. In this embodiment of the invention, the necessary additional steam is thus provided by HP steam.
According to a third aspect to the invention, there is provided an integrated process for producing synthesis gas and energy, which process includes
in a synthesis gas production stage, reforming a hydrocarbonaceous gaseous feedstock to a synthesis gas comprising at least CO, H2 and CO2;
in an electricity generation stage comprising a gas turbine generator, burning a combustible gas in a combustion zone or chamber of the gas turbine generator, to form combusted gas, and expanding the combusted gas through an expansion chamber of the gas turbine generator to form hot flue gas, while generating electrical energy by means of the gas turbine generator;
in a heat exchange stage, using hot flue gas from the electricity generation stage to heat water and/or steam to produce high pressure (xe2x80x98HP steamxe2x80x99) and/or superheated HP steam having a pressure between 3000 kPa(a) and 12000 kPa(a); and
feeding at least a portion of the HP steam to the synthesis gas production stage at process steam.